1. Field of the Invention
This invention relates to stoving enamels and, more particularly, to stoving enamels containing blocked isocyanates.
2. Description of the Prior Art
In the production of mixtures of polyisocyanates and polyhydroxy compounds that can be stored at normal temperatures, it is known to use polyisocyanates whose reactive groups are protected by reaction with monofunctional compounds and which are generally designated as blocked, capped or masked polyisocyanates. Such products are described in Liebigs Annalen Vol. 562, p. 205-229. At elevated temperatures these compounds are cleaved into their original constituents, allowing the liberated isocyanate groups to react in turn with the polyhydroxy compounds present in the reaction mixture.
Such a single component stoving enamel system that is stable enough to store at room temperature consists typically of an .epsilon.-caprolactam-blocked adduct of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate dissolved in a high boiling enamel solvent and a polyester containing hydroxyl groups based on predominantly aromatic dicarboxylic acids and a mixture of diols and triols. Such an enamel is described in German OS No. 23 46 818. The curing rate of such a system is, however, too slow for certain applications. It is possible by using catalysts such as tertiary amines or organic tin compounds to increase the curing rate considerably. However, such additives react with the capped polyisocyanates even at cold temperatures, and particularly in the presence of compounds that can react with . isocyanates, so as to cause the monofunctional protective compound to split off even at normal temperatures and thereby make the mixture unstable.
Another disadvantage of this single component stoving enamel system described in German OS 23 46 818 is the high concentration of the blocking agent liberated during curing.
The recited disadvantages are largely eliminated by the present invention, i, e., curing time, curing temperature, and the amount of blocking agents split off are reduced.